Conventionally, inkjet printing apparatuses have widely been used for a printer, copying machine, and the like because of low noise, low running cost, and easy downsizing of the apparatus. Of these inkjet printing apparatuses, an inkjet printing apparatus has recently been popular, which uses thermal energy as energy used to discharge ink and discharges ink by bubbles generated by thermal energy.
In the inkjet printing apparatus, when a foreign matter such as an unwanted ink droplet or paper dust attaches to an orifice surface (printhead end face which has orifices and faces a printing medium), the ink discharge direction deviates, the ink droplet landing position shifts, and the image quality decreases. That is, the inkjet printing apparatus prints by discharging ink droplets from the printhead to a printing medium (e.g., a paper sheet or OHP film). Small ink droplets may attach to the orifice surface of the printhead due to small ink droplets formed other than discharged main ink droplets or the splash of ink droplets landed on a printing medium, and the orifice surface may become wet. Small ink droplets formed by small ink droplets other than main ink droplets upon discharge or the splash of ink droplets are called an ink mist or simply a mist. When the orifice surface gets wet by ink and a large amount of ink is deposited around the orifice, ink discharge may be inhibited to discharge ink in an unexpected direction (distortion), or no ink droplet may be discharged (non-discharge).
To solve these problems caused by the use of liquid ink in the inkjet printing apparatus, a water repellent is formed on the face surface (orifice surface) in an inkjet printhead to repel ink droplets around the orifice, thereby preventing non-discharge and distortion. As a unique arrangement which is not adopted in other printing apparatuses, the inkjet printing apparatus employs an arrangement in which a wiping member in contact with the orifice surface is arranged and the wiping member and orifice surface are relatively moved to wipe a foreign matter such as ink droplets on the orifice surface. This arrangement refreshes (recovers) the orifice surface to prevent or recover distortion of the discharge direction or non-discharge. The wiping means often adopts an arrangement in which the orifice surface is wiped by a blade (wiper) formed from an elastic material such as rubber, thereby wiping unwanted ink droplets. As for the timing when the wiping means is performed, a deposit on the orifice surface is generally removed during printing or at the end of printing.
Japanese Patent Laid-Open No. 2000-094701 (U.S. Pat. No. 6,283,574) discloses an arrangement in which the wiping frequency during printing of one page of the next printing sheet is decreased by controlling to execute wiping operation under predetermined conditions after delivery of a printing sheet in order to reduce density unevenness upon a change in printing density caused by wiping operation within one page of the printing sheet.
However, the conventional inkjet printing apparatus suffers peeling of the water repellent formed on the head surface along with an increase in wiping count, or a short service life of the head due to the wear of the face surface.
Along with recent reductions in the size and cost of inkjet printing apparatuses, printing element substrates (semiconductor chips) on which orifice groups and orifice lines are formed are being downsized. Further, as printing apparatuses achieve high image quality, the drop size is decreased to eject smaller ink droplets, and orifices are arranged at higher density. With compact printing element substrates and high orifice arrangement density, problems which have been negligible in conventional inkjet printing apparatuses become significant. These problems will be described in detail.
Ink droplets discharged from a plurality of adjacent orifice groups or orifice lines are considered to be influenced by air flows formed by ink droplets flying from the adjacent orifice groups or orifice lines, compared to ink droplets discharged from a single orifice group or orifice line. More specifically, ink droplets which are discharged from orifices and land on a printing medium generate downward air flows along the loci of ink droplets and air flows which spread around ink droplet landing positions along the printing medium. When a plurality of orifice lines each having an array of orifices exist and ink droplets are discharged from the orifice lines, air flows which are formed along the printing medium by ink droplets from the orifice lines collide against each other, generating upward air flows from the ink droplet landing positions toward the orifice lines. As the interval between adjacent orifice groups or orifice lines decreases along with downsizing of the printing element substrate described above and the orifice density increases, the influence of air flows formed by ink droplets flying from adjacent orifices becomes stronger than in the conventional printing apparatus.
As a result, a mist generated by ink droplets other than main droplets upon discharge or splash upon landing flies up under the influence of air flows, and attaches to the face surface having the orifices of the printhead in accordance with the distance between the orifice groups or orifice lines of the printhead used in the printing apparatus, the discharge frequency, and the ink droplet discharge rate. As the interval between adjacent orifice groups or orifice lines decreases, image errors such as a shift of the ink droplet landing position in a printed image and non-discharge of failing to discharge any ink droplet readily occur in comparison with the conventional inkjet printing apparatus. Even if the interval between adjacent orifice groups or orifice lines decreases, the frequency of performing the orifice cleaning means such as cleaning or wiping for the printhead abruptly increases to obtain a stable image.